This invention relates generally to an improved silicon deposition process and deposition substrate, and more specifically to an improved silicon deposition process utilizing partitioned deposition vessels.
To provide single crystal silicon for the semiconductor industry, it is typical to grow large single crystal ingots of silicon having the proper conductivity type and resistivity. The ingots are grown in a bell jar by depositing the silicon on one or more filaments heated by passing a current therethrough. The ingots are sawed into a plurality of thin circular wafers which are lapped and polished to the desired surface finish and thickness. The final wafers are of the high quality needed for many semiconductor devices such as integrated circuits, transistors, and the like. Producing wafers in this fashion, however, is very expensive. For many semiconductor devices the high quality wafer is necessary and the cost of the wafer is relatively insignificant compared to the high cost of the finished devices. In contrast, with some large area devices such as, for example, photovoltaic cells or solar cells, the cost of fabricating wafers in this fashion is prohibitive, especially when considering that wafers of this high quality are not absolutely necessary to the functioning of the solar cell device. Devices of this type, therefore, need a technique which is less expensive to produce the large quantities of required silicon.
One such technique for producing silicon of quality at least suitable for use in solar cells is the ribbon-to-ribbon (RTR) process. By this process, ribbons of polycrystalline silicon are converted directly to ribbons of macrocrystalline silicon without the need for first growing a single crystal ingot or for the sawing and lapping operations. In this context the term "macrocrystalline" refers to material having sufficiently large grains to permit significant phototovoltaic activity. In the RTR process a sheet of polycrystalline material is converted to macrocrystalline material by sweeping a molten zone across the polycrystalline material, causing the material to freeze out behind the molten zone with enhanced crystal size.
In order to make the RTR process successful, however, it is necessary to first produce sheets of polycrystalline material suitable for the conversion process. To this end, polycrystalline silicon has been deposited, for example by chemical vapor deposition or plasma deposition, onto a refractory substrate and then separated from the substrate by thermal expansion shear separation (TESS). These deposition processes, however, although more efficient than the aforementioned bell jar process, are of generally low chemical efficiency, with a large fraction of the available silicon in the silicon bearing reactant material not being utilized. A need therefore existed for an improved process for producing thin sheets of polycrystalline silicon in an efficient and economical manner.
It is therefore an object of this invention to provide an improved process for depositing polycrystalline silicon.
It is another object of this invention to provide an improved process for the simultaneous production of a plurality of silicon ribbons.
It is yet another object of this invention to provide an improved process for depositing silicon at enhanced chemical and energy efficiency.
It is still another object of this invention to provide an improved deposition substrate.
It is another object of this invention to provide an improved process for depositing silicon layers of uniform thickness.